Patent Document 1 and Patent Document 2 relate to correction of a voltage of an end-of-discharge at which discharge of a sodium-sulfur battery is stopped.
In the correction of the end-of-discharge voltage disclosed in the Patent Document 1, an internal resistance is separated into an ohmic resistance and a polarization resistance, and the product of a discharge current and the ohmic resistance and the product of the discharge current having a time delay process performed thereon and the polarization resistance are subtracted from an end-of-discharge open voltage, with a resulting value being set as the end-of-discharge voltage.
In the correction of the end-of-discharge voltage disclosed in the Patent Document 2, the product of an internal resistance and a discharge current is subtracted from an end-of-discharge open voltage, with a resulting value being set as the end-of-discharge voltage. The internal resistance is corrected based on the number of charge/discharge cycles and a healthy parallel number.